(A) Field of the Invention
The present invention relates to a core of an optical patch cord, a method for preparing the same and an optical signal transmission system using the same, and more particularly to a core of an optical patch cord capable of connecting a light source and a multi-mode fiber with optical lossless and without using a precise coupling apparatus, and a method for preparing the same and an optical signal transmission system using the same.
(B) Description of the Related Art
An optical fiber primarily consists of a core and a cladding enclosing the core. The refraction index of the core is higher than that of the cladding, which allows a total reflection to occur when the light beam propagates from dense medium with a high refraction index to loose medium with a relative lower refraction index. Consequently, the light beam can propagate in the dense medium continuously. The optical fiber can be divided into two types: step index fibers and graded index fibers in terms of the refraction index, or single-mode fibers and multimode fibers in terms of the propagation mode.
Due to the great variation of the refraction index at the interface of the core and the cladding of the step index fiber, the light beam from the core to the cladding will be totally reflected at the interface, and the light beam will propagate continuously in the core through the total reflection. The step index fiber has many propagation modes, and each of them delivers optical signals at different speeds, which will result in signal distortions (i.e. diffusion phenomenon) because of different arriving time to the same destination site. The diameter of the core of a single-mode fiber is extremely thin, which just allows the light beam parallel to the central axis to couple into the core; therefore, the arriving time for the light beam to reach the destination point is the same. However, when the emitting spot of the light source is greater than that of the cross-sectional surface of the core of the single-mode fiber, there will be some optical loss because most of the light beam cannot couple into the core.
The refraction index of the core of the graded index fiber is designed to be a parabolic distribution and decrease with the increase of the radius. The propagation speed of the light beam in a medium decreases as the refraction index of the medium increases. A light beam deviating from the central axis propagates in the medium with a lower refraction index, so it propagates at a higher speed but a longer path; while a light beam at the central axis will propagate in a medium with a higher refraction index, so it propagates at a lower speed but a shorter path. Therefore, the time delay between the arriving times of different propagation modes, i.e., differential mode delay (DMD), depends on the refraction index distribution of the core.
FIG. 1 is a cross-sectional view of a core 10 of a graded index fiber and its refraction index distribution as a function of radial position disclosed in U.S. Pat. No. 6,356,680 B1. The preparation of the core 10 first deposits chemical compounds slowly in a glass tube, and the desired distribution of the refraction index can be achieved by controlling the flow rates of deposited chemical compounds such as a reacting gas with germanium. When the chemicals deposited on the inner wall of the glass tube reaches a predetermined thickness, the delivery of the reacting gas is stopped to form a hollow glass rod. The hollow glass rod is then heated and molten to form a solid preform, which will undergo a spinning process to form the core 10. During the heating and melting process, a portion of germanium in the hollow glass rod become germanium oxide, which is gas and escapes into the air. Consequently, an abnormal refracting region 12 is generated at the center of the core 10.
When a laser beam from a single-mode laser source emits into a core such as the core 10 of a multimode fiber, the abnormal refracting region 12 at the center will result in pulse splitting of light signals. Therefore, when connecting a single-mode laser source with the core 10, it is necessary to avoid the launch of the laser beam of the single-mode laser source into the center of the core 10. U.S. Pat. No. 6,356,680 B1 teaches to set an opaque spot at the center of the core 10 to block the laser beam from coupling into the center of the core 10 so as to prevent the laser beam from propagating along the center of the core 10 to reach the light sensor at the destination. Consequently, the laser beam of the single-mode laser source launching into the core 10 at one end can only reach the light sensor at another end by propagating along the region outside the abnormal refracting region 12, and the pulse splitting resulting from the laser beam propagating along the abnormal refracting region 12 will not occur. In addition, U.S. Pat. No. 6,356,680 B1 also teaches that the opaque spot can preferably block 90% of the energy. However, since the opaque spot at the center blocks the laser beam to launch into the core 10, laser beam irradiating on the opaque spot will cause a 90% optical loss.
Another prior art invention for eliminating pulse splitting uses a precise coupling apparatus to connect a single-mode laser source and a multimode fiber. The laser source can only launch its laser beam into a portion of propagation modes of the multimode fiber, and the precise coupling apparatus is used in practice to launch the laser beam into the multimode fiber uniformly along the radial direction, which allows the laser beam to enter all propagation modes of the multimode fiber uniformly to avoid pulse splitting (see: Cisco systems, “Catalyst 5000 Series Mode-Conditioning Patch Cord Installation Note”). As the precise coupling apparatus must be used to assist the alignment of the laser source and the multimode fiber, it is inconvenient in assembly and design. In addition, using the precise coupling apparatus increases the total cost, which does not comply with the industrial demand.